How can i reduce the size of my shadows?
you could make it so that the background is not the light source with this node setup
Then add a sunlamp, with it you can either increase your lamp's strength or decrease its size

A smaller lamp size will give a sharper shadow



then change it so it matches your background's color and thats it.

here is a more detailed answer to your questionn 
How to minimize/soften the shadows?